1. Field of the Invention
The present invention relates to a projector having a projection mirror for reflecting projection image light generated by an image generating device to project an enlarged image onto a projection surface such as an external screen or the like, and more particularly to a projector which is designed for higher luminance and has a foreign matter detecting means.
2. Description of the Related Art
Projectors for projecting an enlarged view of image light generated by an image generating device onto a projection surface such as an external screen or the like are used in various applications. In recent years, efforts have been made to increase the luminance of images projected by projectors, and, as a result, more and more projectors project highly intensive light to form bright images. Accordingly, such projectors have projection lens that are heated to very high temperatures.
Recently, there has been disclosed a projector having a housing from which projection light is emitted through an opening defined in the housing, and a projection mirror for reflecting the projection light to project an image onto a projection surface.
As shown in FIG. 1 of a schematic side elevational view, projector 50 has housing 51 with opening 52 defined therein. Opening lens 53 for projecting projection light is disposed in opening 52. Housing 51 supports thereon projection mirror 54 disposed in confronting relation to opening lens 53. When projector 50 is in use, projection image light 55 generated by an image generating device is emitted through opening lens 53 from opening 52, and reflected by projection mirror 54 to project an enlarged image onto an external screen (not shown). A light source, the image generating device, and an optical system of projector 50 are omitted from illustration. Projection mirror 54 may also be used as a lid for opening 52, and may be turned down to close opening 52 when projector 50 is not in use.
Japanese laid-open patent publication No. 11-119343 discloses a projection-mirror-type projector as shown in FIGS. 2A and 2B of the accompanying drawings. The disclosed projector comprises light source 64, liquid crystal display device 65, projection mirror 63 for projecting projection light modulated by an image which is displayed by liquid crystal display device 65 onto an external screen or the like, a pair of reflection mirrors 66, 67 for reflecting the projection light from liquid crystal display device 65 toward projection mirror 63, and housing 61 which accommodates therein the projection optical system including light source 64, liquid crystal display device 65, and reflection mirrors 63, 66, 67.
Housing 61 has opening 62 provided therein between liquid crystal display device 65 in housing 61 and projection mirror 63, for emitting the projection light from liquid crystal display device 65 toward projection mirror 63. Projection mirror 63 is angularly movably mounted on housing 61 for selectively opening or closing opening 62. Projection mirror 63 serves as part of the projection optical system when it opens opening 62, and serves as a lid for opening 62 when it closes opening 62.
As described above, with the projector having the projection mirror, the opening lens and the reflection mirrors which are heated to a relatively high temperature are exposed out of the housing. Furthermore, when foreign matter such as a user's finger, an object in the room, or dust particles enters the opening, a sharp temperature rise occurs because the projection light directed toward the projection mirror is blocked by the foreign matter. Since such a sharp temperature rise may possibly cause a burn on the user or a fire in worst cases, some countermeasures need to be combined with the projector.
For example, in case a detecting circuit with a sensor assembly for simply detecting foreign matter that enters the opening is added, huge quantities of sensors are needed. In case the sensor assembly generally detects foreign matter in a wide range of space, there needs a plurality of sensors. Furthermore, as the size of foreign matter to be detected is smaller, the sensor assembly requires an increased number of sensors, and hence the detecting circuit becomes larger in circuit scale.
If the projection-mirror-type projector disclosed in Japanese laid-open patent publication No. 11-119343 is designed for higher luminance, then the intensity of the projection light in the vicinity of the reflection mirrors is increased, resulting in a higher temperature at those reflection mirrors. However, there is nothing disclosed in Japanese laid-open patent publication No. 11-119343 about countermeasures against foreign matter which may possibly find its way into the path of the projection light.